Safety regulations for protecting vehicle passengers and fuel efficiency regulations for protecting the environment have recently been tightened, and thus there is increasing interest in techniques for improving the stiffness of automobiles and reducing the weight of automobiles.
For example, components such as stabilizer bars or tubular torsion beam axles of automotive chassis are required to have both stiffness and durability because they are used to support the weight of vehicles and are constantly subjected to fatigue loads during driving.
Moreover, the weight of vehicles has been gradually increased because of the recent increasing use of comfort components, and thus test conditions for guaranteeing durability have been tightened. Accordingly, the application of ultra high strength steels to heat treatable steel components has been increased for performance improvements and weight reduction.
The fatigue life of steel sheets for automotive components is closely related with the yield strength and elongation of the steel sheets, and the fatigue life of heat treatable steel sheets is affected by surface decarburization occurring during heat treatment processes or surface scratches formed during steel pipe manufacturing processes.
In particular, the influence of these factors increases in proportion to the strength of steel, and thus methods for manufacturing high strength automotive components having a tensile strength grade of 1500 MPa or greater, while solving problems arising during processes of forming ultra high strength steels, have been proposed.
Examples of such methods include a hot press forming method, in which high-temperature forming and die quenching are performed simultaneously, and a post heat treatment method in which cold forming, heating to an austenite region, and quenching by contact with a cooling medium instead of contact with a die, are performed sequentially. However, martensite obtained after quenching has low toughness even though it has high strength. Thus, to improve toughness, a method of performing a tempering process after a quenching process has been commonly used.
The degree of strength obtainable by the hot press forming method or the post heat treatment method is various, and a method of manufacturing automotive components having a tensile strength grade of 1500 MPa, using a heat treated-type steel pipe containing 22MnB5 or boron, was proposed in the early 2000 s.
Such automotive components are manufactured by producing an electric resistance welding (ERW) steel pipe using a hot-rolled or cold-rolled coil, cutting the ERW steel pipe in lengths, and heat treating the cut ERW steel pipe. That is, such automotive components are manufactured by producing an ERW steel pipe through a steel sheet slitting process, performing a solution treatment on the ERW steel pipe by heating the ERW steel pipe to an austenite region higher than or equal to Ac3, and extracting the ERW steel pipe and hot forming the ERW steel pipe using a press equipped with a cooling device such that die quenching is performed simultaneously with the hot forming. In some cases, after the hot forming, hot-formed products may be taken out from a die and may then be quenched using a cooling medium.
In other methods, ultra high strength components having a strength of 1500 MPa or greater and martensite or a mixed phase of martensite and bainite as a final microstructure may be manufactured by cold forming a steel sheet in a shape similar to a component shape, performing a solution treatment on the cold-formed steel sheet by heating the cold-formed steel sheet to an austenite region higher than or equal to Ac3, and extracting the heated steel sheet and quenching the heated steel sheet using a cooling medium, or such ultra high strength components may be manufactured by hot forming a steel sheet in a final product shape by using a die, and quenching the hot-formed steel sheet by bringing the hot-formed steel sheet into contact with a cooling medium.
In addition, a tempering process may be performed to increase the durability life and toughness of the components quenched, as described above.
In general, a tempering process is performed within a temperature range of 500° C. to 600° C. and, as a result of the tempering process, martensite transforms to ferrite, in which cementite is precipitated. Thus, although tensile strength decreases and a yield ratio increases to a range of 0.9 or greater, uniformity and total elongation are improved as compared to a quenched state.
As the weight of automobiles increases, there is an increasing need for higher-grade components made by heat treated-type steel pipes.
In a strengthening method, the content of manganese (Mn) and the content of chromium (Cr) in steel are fixed to a range of 1.2% to 1.4% and to a range of 0.1% to 0.3%, similar to the contents of Mn and Cr in heat treatable steel of the related art containing boron (B), and the content of carbon (C) in the steel is increased as a result of considering post-heat treatment strength of the steel. Based on the strengthening method, however, fatigue cracking and sensitivity to crack propagation increase because of an increase in strength, and thus the durability of steel, that is, the fatigue life of steel, is not increased in proportion to the increase in the strength of the steel.